Air entrainment and bubble dispersion were studied systematically at vertical circular plunging jets in freshwater, saltwater and seawater. Although the inception conditions for air entrainment are identical for all water solutions, experimental results show lesser air entrainment in seawater plunging jets for identical inflow conditions. Comparison between seawater and saltwater results suggest that organic matter and bio-chemicals harden the induction trumpet at plunge point and diminish air entrapment at impingement in seawater. Froude-similar experiments with different geometric scales show lesser air entrainment for inflow conditions satisfying We < 1E+3. The results show that classical dimensional analysis of air entrainment is incomplete, while they would imply scales effects in terms of air entrainment for small laboratory studies with wave height at breaking less than 0.3 m.